Sublimation, enantiomers

RS- P-Aminoisobutyric acid (a-methyl-P-alanine) [10569-72-9] M 103.1, m 176-178 , 178-180 , 181-182 , R -(-)- isomer [144-90-1] m 183 , [a] -21 (c 0.43, HjO), pKes,(,) 3.7, pKEst(2) 10.2. Colorless prisms from hot H O, were powdered and dried in vacuo. The purity is checked by paper chromatography (Whatman 1) using ninhydrin spray to visualise the amino acid Rp values in 95% MeOH and n-PrOH/5N HCOOH (8 2) are 0.36 and 0.50 respectively. [Kupiecki and Coon Biochem Prep 7 20 7960 Pollack J Am Chem Soc 65 1335 7943.] The R-enantiomer, isolated from iris bulbs or human urine was crystd from H2O and sublimed in vacuo [Asen et al. J Biol Chem 234 343 7959]. The RS-hydrochloride was recrystd from EtOH/Et20 m 128-129 , 130° [Bbhme et al. Chem Ber92 1258, 1260, 1261 7959]. 

Sublimation. Such effects can also be seen in solid-gas interphase. Scheme 30 shows the consequences of fractional sublimation of partially resolved L-mandelic acid (47). The optical purity could be enhanced or reduced, depending on the optical purity of the starting material. Since the eutectic point of mandelic acid is obtained with about a 75 25 enantiomer ratio, such a mixture is more readily sublimed than the racemate or conglomerate. Scheme 30 gives other examples of optical enrichment by sublimation. Phenyl 1-phenyl-1-propyl sulfide in 6% ee affords the sublimed compound in 74% ee, but the residue is... 

Mandelic acid sublimation, 280 Mandelonitrile, 233, 330 Melting point, enantiomers, 279 Menthol, 9... 

After filtration, the alcohol enriched in one enantiomer was obtained from the filtrate, the other one was isolated from the solid complex. Thermal methods such as distillation or sublimation were used for liberation of the optically active alcohols from the complexes. [38] Time-scaled experiments showed continous increasing of the enantiomeric excess (ee) in the crystalline diastereoisomeric complexes of 26 and 8 for 14 days, then these ee values practically did not change within the next 16 days (Figure 1). 

Solvent free LnL complexes (Type I) resulted from the silylamide route [184]. Crystals of Yb(trac) could be grown by sublimation (180 °C/10 2 Torr). The heptadentate ligand is wrapped around the ytterbium emerging in a disordered geometry. Both enantiomers, the right- (A) and left-handed (A) screw, are present in the unit cell. 

This book illustrates the recent aspects of amplification of chirality by asymmetric auto catalysis and by forming helical structures. The first four chapters summarize experimental asymmetric autocatalysis with amplification of enan-tiopurity, the mechanism of asymmetric autocatalysis examined by NMR and calculation, the computer simulation models of the reaction mechanism of asymmetric auto catalysis, and the theoretical models of amplification of chirality. The last chapter deals with the amplification of chirality by the formation of helical structures. However, the amplification of enantiopurity in non-auto catalytic asymmetric reaction and the amplification by enantiomer separation involving crystallization or sublimation are beyond the scope of this book. 

A phenomenon that may be related to the origin of biological homochirality was recently reported by Cooks and coworkers 39 Serine sublimes with spontaneous chiral amplification. Sublimation of near racemic sample of serine 55 (Figure 9) yields a sublimate that is enriched in the major enantiomer. The chiral purity maximizes at 190-210 °C, and then falls as thermolysis becomes favorable. This simple one-step sublimation may represent a possible mechanism for the chiral amplification step to explain the origin of biological homochirality. 

Pentobarbital (5-ethyl-5-l -methylbutyl barbituric acid. Nembutal is the Na salt) [76-74-4] M 226.4, m 127 (dec), pKesj )- 8.0, pKEst(2) 12.7. A solution of the sodium salt in 10% HCl is prepared, and the acid is extracted with ether. Evaporation of the extract gives a solid which is then purified by repeated crystallisation from CHCI3. It sublimes at 95-105 /10-12mm. [Bucket Sandorfy J Phys Chem 88 3274 1984 The f-i-)- and (-)-enantiomers crystallise from 50% aqueous EtOH with m 120-121° and have [a] h-4.73° to. 93° (EtOH) [Kleiderer Shonle / Am Chem Soc 56 1772 1934]. [Beilstein 24 I... 

The separation of C o may further be attained by absorptive filtration on a mixture of charcoal powder and siUca gel. Pure toluene can be used here which distinctly increases the solubiUty, thus accelerating the process and markedly reducing the solvent consumption. Apart from Q,q, it has by now become possible to obtain C70 on a multigram scale as well as to make higher fullerenes available at least in amounts sufficient for physical examination. Highly purified materials can be produced by sublimation of the above substances. Chiral fullerenes (refer to Section 2.2.3) may be split into enantiomers by suitable methods, for example. 

Vapor Pressure. Differences in vapor pressure (i.e., in the heat of sublimation [Ai/subiim]) have been observed for enantiomers and racemates. It has been suggested that heterochiral interactions between solid enantiomers (as in a racemic compound) can be either stronger or weaker than the homochiral interactions, i.e., between the crystals of the 1 1 mechanical mixture [32]. These interactions have not been widely investigated in pharmaceutical formulations containing racemates or enantiomers. The difference in A/fsubiim can also be used to separate enantiomers from a mixture, and it is thought to be a better method of separation, at least in some cases, than recrystallization. 

Sublimation (section 8.3) could have a role to play in the separation of enantiomers since the vapour phase arising from a soHd mixture of enantiomers is always racemic, irrespective of the ratio of enantiomers present in the solid. Therefore, if the starting solid contains unequal amounts of enantiomer, sublimation will increase the purity of the major component in the residual solid phase. There does seem to be a possibihty for improving the purity of near-pure enantiomers, e.g. those that had been partially resolved by other methods. However, as only a few experimental studies of enantiomer separation by sublimation have so far been recorded, the technique must still be regarded as speculative (Jacques, Collet and Wilen, 1981). 

A racemate can be resolved with ease if it happens that the enantiomers form separate crystals - a so-called conglomerate. It becomes possible to separate, physically, the enantiomorphic crystals - a process sometimes referred to as triage. This is what Pasteur famously achieved in 1848 with a sample of ammonium sodium tartrate [6]. Such good fortune is quite rare and is in any case not a synthetic method in the strictest sense (nor is it practical on a large scale). Other physical processes (alone, without any attendant synthetic process) such as evaporation or sublimation can be used to increase the enantiomeric excess of organic compounds, including amino acids [7]. 

However, the mode of separation of the crystalline fraction is very important. For example, the separation of enantiomers can be effectuated even if a mixture from the melt of enantiomeric mixtures is crystallized. The crystalline material can be separated from the residue by filtration, distillation, sublimation or extraction. ... 

Sublimation method for indirect separation of enantiomers is very similar, in principle to the above mentioned distillation-based procedure. In the next example the diastereoisomeric molecular complexes could be separated using the significant difference between their thermal stability. 

The solid phase reaction between the racemic compound (fj flns2-2-iodo-cyclohexanol = IC)) and the resolving agent ((R,R)-DBTA) occurs during a long-term staying.27 Then the enantiomers could be separated from the mixture by two sublimation steps. 

---